The present invention relates to a method and to apparatus for biological purification of waste water, of the type in which activated sludge (i.e. a bacteria culture dispersed in the form of flakes) is caused to develop in water treatment tanks, and in which, after a sufficient period of contact, the purified water is separated from the sludge by sedimentation (in a clarifier), with the sludge being recycled in one of the treatment tanks in order to maintain a sufficient concentration of purifying bacteria therein, while the surplus, excess secondary sludge, is removed from the system. This type of purification method seeks both to eliminate organic carbon pollution by oxidation using (free or bonded) oxygen dissolved in the water, and also to eliminate nitrogen pollution in all its forms (proteins, amino acids, urea and decomposition products, and nitrogen in inorganic form, in particular ammonia salts) by nitrification or by nitrification-denitrification, with nitrate reduction taking place prior to nitrification.
It has been observed that in purification methods making use of activated sludge, the limiting factor for nitrification is not the kinetics of the transformation of NH.sub.4 into NO.sub.3 (or of organic N into NO.sub.3) but rather the age of the sludge, i.e. the actual time the sludge stays in the system. The limiting age is the age that needs to be adopted in order to conserve the nitrogen-fixing microorganisms in the purification station, and this is known to be very high because of the very long regeneration time of nitrogen-fixing microorganisms; as a result, it is necessary to provide very large volume aeration tanks which are over-dimensioned relative to reaction kinetics.
Several solutions have been proposed for diminishing these volumes:
One proposal, for example, has been to increase the concentration of the biomass being aerated: if the age of the sludge is written A, the volume being aerated V, the average concentration of the biomass X, and the daily production of excess sludge .DELTA.X, then the following equation can be written: EQU A=VX/.DELTA.X
Assuming that .DELTA.X is constant, V can be made smaller by increasing X. However, in a conventional purification station, the upper limit for X is rapidly attained, since at a biomass concentration of more than 5-6 grams per liter (g/l), the capture ratio of the clarifier becomes too low to produce effluent of acceptable quality.
In order to obtain a further increase in the concentration of the biomass being aerated, a system has been proposed comprising two separate tanks: a "contact" tank in which ammonia is transformed into nitrates (nitrification); and a "stabilization" tank for increasing the age of the sludge. In this case, the contact tank has a concentration of material in suspension which is compatible with clarification for obtaining effluent of acceptable quality; as a result the stabilization tank can operate with sludge at a concentration higher than that in the contact tank, with said concentration being equal to the concentration of the recycled sludge. The concentration of recycled sludge is directly related to the recycling fraction. If the commonly-adopted recycling fraction of 100% of the raw water throughput is assumed, then a recycled sludge concentration is obtained which is equal to twice the concentration of the sludge in the contact basin, i.e. about 10 g/l. However, this increase in sludge concentration in the stabilization tank is nevertheless not very much greater than the maximum concentration which can be used in a conventional purification station.
The reaction tank, or contact tank, is a tank in which the depollution reaction occur.
The generation tank, also known as the stabilization tank, is a tank in which the sludge ageing stage takes place.
The initiation tank is the tank in which the initiation stage of the reactions occurs, particularly dephosphatation, the these reactions being effectively realized in the reaction tank.
The biological reactions taking place in water treatment can be characterized by two magnitudes:
R which represents substrat degradation kinetics; and PA1 .mu. which represents microorganism growth rate under the conditions prevailing in the station. PA1 Q=flow rate of the effluent to be treated; PA1 C=concentration of pollution to be eliminated (kg per m.sup.3); PA1 R=specific pollution degradation rate (kg per kg of matter in suspension (MeS) per day); and PA1 Xr=microorganism concentration in the reaction tank (kg of the MeS per m.sup.3); PA1 .mu.=the microorganism growth rate; PA1 A=1/.mu.=the sludge age (in days) necessary for obtaining the reaction; and PA1 Xg=micro-organism concentration in the generation tank (kg of MeS per m.sup.3); PA1 A=sludge age necessary for obtaining the reaction (in days); PA1 Px=daily production of sludge (kg of MeS per day); PA1 Xg=sludge concentration in the generator tank (kg of MeS per m.sup.3); PA1 Xr=sludge concentration in the reaction tank (kg of MeS per m.sup.3); PA1 X=sludge concentration at the inlet to the clarification equipment (kg of MeS per m.sup.3); PA1 Q=installation throughput; PA1 C=concentration of pollution to be eliminated (kg per m.sup.3); and PA1 R=specific pollution degradation rate (kg per kg of MeS per day).
It is also possible to characterize the volumes of the reaction tank and the generator tank by various parameters. For the reaction tank, and using the following symbols:
then the volume Vr of said tank can be obtained from the following equation: EQU Vr=CQ/RXr
For the generator tank, and using the following variables:
by definition, .mu.=(1/X)(dX/dt), where X is sludge concentration in kg of MeS per m.sup.3 at the inlet to the clarification works; which, under steady state conditions, becomes: EQU .mu.=(1/X)(.DELTA.X/.DELTA.t),
giving a daily production Px of sludge equal to: EQU Px=V(.DELTA.X/.DELTA.t)
(where V represents the maximum volume of the reaction tank and the generator tank); in which case the following equation defining the age of the sludge can be written: EQU A=XgVg/Px (1)
from which the volume of the generation tank Vg can be deduced as follows: EQU Vg=APx/Xg
With microorganism generation taking place in this way in the reaction tank, the overall equation for the age of the sludge can be written as follows: EQU A=(XgVg+XrVr)/Px (2)
The total volume of a water treatment system can thus be obtained by the following equation: EQU V total=Vg+Vr
Further, since items A (age of the sludge) and Px (daily sludge production) are defined by the pollution to be eliminated and by the quality of the effluent to be treated, the parameter which governs proper operation of the water treatment installation is the weight of sludge being aerated, which is given by multiplying V by X.
In a conventional system, said weight of sludge being aerated is, by definition, given by the following equation: EQU A=VX/Px, i.e. VX=APx (3)
(where VX corresponds to the weight of sludge, since V is the volume being aerated and X is the microorganism concentration in the tank).
Finally, from equation (2) above, the following equation can be written: EQU (XgVg+XrVr)=APx (4)
and combining equations (3) and (4), the following equation can be written: EQU VX=VgXg+VrXr
It is consequently highly advantageous to be able to provide a method and apparatus capable of increasing the concentration of activated sludge in a water purification station, with the concentration obtained being substantially greater than the concentrations obtained by prior art methods, thereby making it possible to considerably reduce the volumes of the treatment tanks in such a station.
The present invention also seeks to provide a method and apparatus which are particularly suitable for eliminating carbon pollution and nitrogen pollution, and for biological dephosphating.